Smartphone Upgrades Slow as 'Wow' Factor Fades

Fewer people are upgrading their smartphones—a trend that could make it harder for companies from AT&T Inc. to Apple Inc. to keep up the pace of revenue growth.

The rates at which American cellphone users have traded in their devices for more advanced models have declined over the last few years, according to analysts at UBS AG. They turned negative last year, when about 68 million people upgraded their phones in the U.S., down...